1. Field of the Invention
The present invention relates to a drive control apparatus for an ultrasonic motor having a driving vibrator with a piezoelectric element and an elastic element.
More particularly, the invention relates to a drive control apparatus for an ultrasonic motor comprising a vibrator acting as a stator or rotor and including a piezoelectric element and an elastic element fixed together, and a rotor or stator in pressure contact therewith, for producing torque with action of a traveling wave generated with the elastic element by high frequency power supplied to the piezoelectric element, or for a linear ultrasonic motor comprising piezoelectric elements provided at opposite ends of a columnar elastic element and a slider slidable along the elastic element.
2. Description of the Prior Art
Conventionally, this type of drive control apparatus for an ultrasonic motor comprises a frequency control device for controlling oscillation frequency of a power supply device. The frequency control device includes a detecting piezoelectric element provided separately from a vibrating piezoelectric element on the elastic element for detecting a vibrating state of the vibrator. An output voltage corresponding to a vibrating amplitude of the vibrator is detected and maintained at a predetermined level.
In order that the ultrasonic motor operate efficiently and steadily regardless of variations in the mechanical resonant frequency of the elastic element due to variations in use environment and conditions such as temperature, load and drive voltage, it is necessary for the vibrating state of the vibrator to follow the resonant frequency.
For this purpose, the drive control apparatus must include a state quantity detecting device for detecting the vibrating state of the vibration or certain quantities of states such as displacement, velocity, acceleration and the like. Output of the detection provides the basis for maintaining source frequency at an optimal frequency for to the piezoelectric element.
According to the prior art noted above, however, it is necessary for the elastic element to have a detecting piezoelectric element separately from a vibrating piezoelectric element in order for the vibrating state of the vibrator to follow the resonant frequency. Besides, a wiring for taking out its signal is also needed. This has the disadvantage of not only complicating the motor manufacturing process but raising the cost of the motor.
Furthermore, such a detecting piezoelectric element, depending on dispersion of its characteristics and location of installation, does not always provide accurate detection of the vibrating state of the elastic element. Consequently, it is difficult to effect controls for an optimal operating state.
The prior art also has the following problems with regard to constant velocity amplitude control.
For speed control of an ultrasonic motor, it has been one conventional practice to control output frequency of a power source that supplies power to the piezoelectric element, to maintain at a predetermined level an output signal of a sensor such as an encoder for detecting rotational speed of the rotor.
In this case, however, the ultrasonic motor needs a sensor such as an encoder for detecting rotational speed of the rotor. Where the sensor is mounted inside the ultrasonic motor, the construction is complicated and a wiring is needed for taking out its output. This has the disadvantage of not only complicating the motor manufacturing process but raising the cost of the motor.
Even where the sensor is provided separately from the ultrasonic motor, a component is required for attaching the sensor to the output shaft of the ultrasonic motor in addition to the sensor itself such as an encoder. This again results in an increase in cost, and a detrimental effect on the simplicity of construction which is the characterizing feature of the ultrasonic motor.
The piezoelectric element of the ultrasonic motor has a drawback that the vibrating amplitude of the vibrator is greatly variable with load variations of the ultrasonic motor. It has been found that the amplitude of the vibrator increases at a time of light load to the extent of damaging the motor, and decreases at a time of heavy load such that sufficient mechanical work cannot be carried out. However, no compensatory control for such load variations is provided in the prior art, resulting in great variations in output characteristics due to the load variations.